Various mechanical devices have long been used to determine when the level of material in a tank or other vessel reaches a certain point. Floats and displacers rely upon the buoyant forces of a liquid, while other devices use diaphragms or vibrating elements to sense the presence of a liquid or solid. Such mechanical devices generally have a high maintenance cost due to deterioration or binding of various mechanically moving parts.
Sensing devices which rely upon measuring the weight of the tank contents require precise knowledge of the specific gravity of the material in order to accurately determine the level of material in the vessel. Optical sensors generally require an extremely clean environment for reliable operation, while sonic devices do not function well in foam or dust environments. Finally, conductivity sensing devices cannot be reliably used for determining the level of many solids or liquids in a tank, such as dielectric materials, and capacitive devices generally are restricted to detecting the level of nonconductive materials.
Microwave sensors have a significant advantage over the devices described above in that such devices may be universally employed for detecting the level of almost any solid or liquid, regardless of its conductivity or specific gravity. Moreover, microwave sensors are generally insensitive to dust, vapors, foam layers, or viscous liquid coatings or thin layers of powder on the sides of the vessel.
U.S. Pat. No. 4,107,993 discloses microwave techniques for detecting the level of a liquid in a vessel. An external chamber constructed of material invisible to microwaves is required, and the system detects amounts of unabsorbed energy to the receiver. Microwave devices of this type experience alignment problems since the transmitter and receiver must be properly positioned with respect to one another.
U.S. Pat. Nos. 4,218,678, 4,359,902, and 4,044,355 also disclose microwave sensing devices for determining the level of materials in a tank. Microwave devices which utilize radar technology generally seek to determine the travel time of a signal to the detected material and thence to the receiver. The expense and complexity of these devices limits their practical use to situations in which the actual level of the material in the tank must be determined, as compared to devices which simply determine whether the material has or has not reached a certain level.
U.S. Pat. Nos. 3,572,119 and 4,458,530 are similarly directed to devices intended to quantitatively determine the level of liquid in a vessel. A sensor monitors the alteration of the standing wave passing through the liquid to determine the liquid level.
The disadvantages of the prior art are overcome by the present invention, and improved methods and apparatus are hereinafter described for inexpensively yet reliably determining whether a solid or liquid material in a container has obtained a certain level or if such material is flowing or not.